Preliminary investigation for quali-quantitative characterization of soils contaminated with 241Am and 152Eu by low-altitude unmanned aerial vehicles (UAVs) equipped with small size γ-ray spectrometer: detection efficiency and minimum detectable activity (MDA) concentration assessment

Localization and quali-quantitative characterization of radionuclide-contaminated soils are essential for healthcare and remediation activities. However, characterization activities are at the moment mainly based on very complex activities, generally dangerous for workers' health. Unmanned aerial vehicles (UAVs) can be considered for this purpose, but the absence of data does not permit their use for a quantitative analysis. The main goal of this work is to obtain scientific information for successfully using UAVs in quali-quantitative characterization activities. The influence of the main flight UAV condition parameters on the detection efficiency and minimum detectable activity (MDA) concentration was assessed at lab-scale, considering Am-241 and Eu-152 as simulated soil contaminants. Obtained data were compared with reference to clearance levels. Specifically, the altitude (h) and inclination (theta) with respect to the soil surface of a small size gamma-ray semiconductor CdTe detector and detection time were investigated as flight parameters. h and theta were adopted in the range 0-70 cm and 0A degrees-60A degrees, respectively. Main results reveal that in the case of Am-241 contamination, a duration time of about 13 and 20 min is required for an altitude of 60 and 70 cm, respectively, if an inclination of zero is considered, whereas inclinations higher than 15A degrees lead to much longer times, up to 40 min. In the presence of Eu-152, results reveal a more marked influence of the variables h and theta on the minimum detection time, with values higher for Am-241. In terms of MDA, in the case of Am-241, all the concentration values, with the exception of the highest theta investigated (60A degrees), are below the Italian regulatory limit for altitudes lower than 35 cm and a detection time of 30 min. Higher h up to 50 cm can be considered, reducing the inclination up to 0A degrees, whereas even higher altitudes make the detection of Am-241 not possible. On the other hand, for Eu-152, the observed MDA values always make the contaminant detection possible. All achievements and the calculated fitting parameters are useful preliminary data in the quantitative characterization of soil surface contamination, representing a valuable tool in knowing the soil surface gamma-ray activity of Am-241 and Eu-152 for each given detected value. In general, obtained results will allow a better understanding of the technique limits and operating methods required for characterization activities by UAVs with vertical take-off and landing system.